As many drops as it takes to have liquid pooling a bit on top! For a regular 510 cart with stock fluval, this may be around 12 -15 drops. The mega cats hold about 20% more I think.
However, stock fluval will hold on to some of the
juice and not wick it fully too the atty, so when you think you need to refill it, it will probably need less than 12-15 drops to do this (this is called topping off).